ISPC 3.1 cant renew LE certs

I've setup a perfect server with Ubuntu 16.04, apache 2.4, MariaDB 10, ISPC 3.1 etc.
ISPC is running several websites without problems for some time now, except for this:

After a while I added a ssl certificate to some of them, using the LE checkbox in ISPC.
All worked great until the certificates had to be renewed.

LE says all domains can't be verified in ISPC log.
When running from the command line LE says <domain>/.well-known/acme-challenge/xxxxxx can't be found.
When I place a testfile in /usr/local/ispconfig/interface/acme/.well-known/acme-challenge I can open it just fine.
So why can't LE find its files?
I've checked the LE and ISPC logs but can't find anything relevant to the cause of the problem.

I've setup a perfect server with Ubuntu 16.04, apache 2.4, MariaDB 10, ISPC 3.1 etc.
ISPC is running several websites without problems for some time now, except for this:

After a while I added a ssl certificate to some of them, using the LE checkbox in ISPC.
All worked great until the certificates had to be renewed.

LE says all domains can't be verified in ISPC log.
When running from the command line LE says <domain>/.well-known/acme-challenge/xxxxxx can't be found.
When I place a testfile in /usr/local/ispconfig/interface/acme/.well-known/acme-challenge I can open it just fine.
So why can't LE find its files?I've checked the LE and ISPC logs but can't find anything relevant to the cause of the problem.